The present invention relates to a method for the corrosion-protective pretreatment of metal components, which at least partially comprise metal surfaces made of iron, using a chromium-free aqueous treatment solution which contains fluoro complexes of zirconium and/or titanium and phosphate ions in a specific ratio range to one another, and a metal component which is pretreated accordingly, and the use thereof for the application of further corrosion-protective coatings and/or lacquer systems. The method is suitable in particular as a pretreatment for electrophoretic painting of metal components, which are present in the form of non-closed hollow bodies. The present invention therefore also provides a method for coating a non-closed metal hollow body which encompasses both the pretreatment using the chromium-free aqueous treatment solution and also subsequent electrophoretic painting, and a metal hollow body which is coated in accordance with the method according to the invention, and the use thereof for the production of radiators.
The passivation of metallic materials, particularly of iron and ferrous steels, is primarily ensured by zinc or iron phosphating. In zinc or iron phosphating, mainly crystalline inorganic coatings are produced on the metal base material, which have a layer thickness of several micrometers and, owing to their surface topography, possess excellent adhesion to organic top coats, especially to lacquer systems applied by an electrophoretic method. In non-film-forming iron phosphating, the conversion of the metal surface is typically carried out in a phosphoric acid medium, and also in the presence of accelerators and wetting agents at an elevated bath temperature. These iron phosphate films seldom have coating weights of more than 1 g/m2 and, in contrast to phosphating with high coating weights, they are amorphous. Classic phosphating usually constitutes a multi-step method consisting of a cleaning step for degreasing the component, an activation process and finally the actual phosphating, with rinse steps being incorporated into the continuous operation to decouple the process baths. A rinse operation of this type is compulsory at least after the cleaning step, so the phosphating is composed of at least four individual processes which have to be monitored and controlled from a process engineering viewpoint in individual baths. These high process engineering requirements and the associated complexity of a phosphating operation sometimes represent an obstacle to the introduction of a passivation of this type for components in low-cost applications outside of car manufacturing. Another technical disadvantage is the processing of residues such as phosphate sludge contaminated with heavy metals, which are unavoidable with the high phosphate contents in the passivating dipping bath and can only be processed with further energy and matter conversion. Overall, therefore, not least the elevated bath temperatures make classical phosphating a method having high energy costs and a huge requirement for recovery measures.
Besides non-film-forming iron phosphating, additional alternative methods to standard phosphating, which yields coating weights of significantly more than 1 g/m2, are conversion treatments of the metal surfaces, forming purely amorphous, inorganic passive layers with much lower coating weights of the order of magnitude of less than 200 mg/m2 in some cases.
All pretreatment methods which bring about such a “non-film-forming” (non-crystalline) phosphating and/or conversion of the metal surface have the advantage that activation of the surface becomes superfluous and thus can be cut from the pretreatment process chain. Another advantage over film-forming zinc phosphating is the reduction of phosphate sludge in the phosphating baths.
For example, U.S. Pat. No. 5,356,490 and WO 04/063414 teach phosphate-free and chromium-free aqueous treatment solutions containing zirconium and/or titanium compounds which are deposited on the metal component in an acidic medium as a so-called passivating conversion coating owing to the pickling attack on the treated metal surfaces. Both documents teach that dispersed water-insoluble inorganic compounds must additionally be contained to achieve the desired effect in terms of corrosion protection and paint adhesion, with WO 04/063414 explicitly requiring the presence of acid-stable, nano-dispersed compounds based on silica and, in contrast to U.S. Pat. No. 5,356,490, managing without the addition of organic polymers.
DE 1933013 also discloses phosphate-free treatment baths having a pH greater than 3.5, which, in addition to complex fluorides of boron, titanium or zirconium in quantities of 0.1 to 15 g/l, based on the metals, additionally contain 0.5 to 30 g/l oxidizing agent, particularly sodium m-nitrobenzenesulfonate. According to the teaching of DE 1933013, the oxidizing agent sodium m-nitrobenzenesulfonate is assigned the function of varying the treatment period of the metal surfaces to a particularly great extent.
In contrast, WO 03/002781 discloses pretreatment solutions which, besides phosphoric acid, also contain fluoro complexes of zirconium and/or titanium and a homo- or copolymer of vinylpyrrolidone. Such a pretreatment solution yields amorphous mixed organic/inorganic passivations with a low coating weight, which can be provided with an electrophoretic paint.
DE 2715292 discloses treatment baths for the chromium-free pretreatment of aluminum cans, which contain at least 10 ppm titanium and/or zirconium, between 10 and 1000 ppm phosphate and a quantity of fluoride sufficient to form complex fluorides of the titanium and/or zirconium present, but at least 13 ppm, and have pH values between 1.5 and 4.
Published patent application US 2007/0068602 discloses a passivating pretreatment solution which, in addition to fluoro complexes of zirconium and phosphate anions, also contains oxo anions of vanadium, the contents of which have to be within a specified ratio range to one another in order to achieve effective corrosion protection.
However, no teaching can be taken from the prior art on passivating pretreatment with compositions containing compounds of zirconium and/or titanium and phosphate regarding which specific compositions of such pretreatment solutions guarantee optimum corrosion protection with optimum electrophoretic paintability of the amorphous passivation layers. For original equipment manufacturers in particular, comparatively low paint consumption with good paint throwing power and equal corrosion resistance of the coated metal component are economically important.